Recently, as a countermeasure against the eutrophication of lakes, ponds and the like, it has become necessary to remove from the waste water phosphoric acids such as polyphosphoric acid, orthophosphoric acid, methaphosphoric acid, pyrophosphoric acid and the like or salts thereof and nitrogen before the waste water is expelled into these bodies of water.
In order to effect dephosphorization, prior art methods have included coagulation and sedimentation processes, biological dephosphorization processes, crystallization-dephosphorization processes, and the like. Since denitrification is also desired, a biological denitrification process has been widely practiced.
The above-described processes of dephosphorization, although having some success, also are plagued with drawbacks. In the coagulation and sedimentation process, stability in the treatment of dephosphorization can be obtained, however, it is difficult to feed a coagulant at a suitable flow rate, whereby a large quantity of the coagulant is needed. Consequently, running cost is high, the operational control becomes troublesome, and further, production of sludge is increased and dewaterability is worsened.
Furthermore, although biological dephosphorization by activated sludge processes or biofilter processes, is seemingly economical due to fluctuations in the flow rate of the waste the process becomes inefficient when the flow rate of waste water varies. Furthermore, in order to prevent phosphorus from being released again from the microorganisms in the excess sludge, chemically coagulated dephosphorization is necessary, thus the treatment process becomes complicated and uneconomical. Furthermore, when the nitrification treatment is combined with the above discussed processes, the dephosphorization rate is adversely affected by the conditions of the denitrification treatment; thus the dephosphorization treatment becomes unstable.
Further, in connection with the crystallization-dephosphorization process, many conditions such as decarbonation and pH control are required in order to provide an effective treatment. The processes are cost and labor intensive.
The present invention contemplates solving the problems concerning dephosphorization, or dephosphorization and denitrification by performing the activated sludge processes including the conventional activated sludge process, the extended aeration process, the intermittent cyclic aeration process, the lagoon process, the oxidation ditch process and the like, and also performing the waste water treatment processes by the biofilter processes including the rotating biological contactor process, the contact aeration process, the fluidized bed process and the like.